Many Londoners were awoken by a loud “bang” at around 4:15 a.m. on Monday morning, which prompted many to take to social media, post what they heard and look for  answers.

The massive sound left many worrying that London may have been the scene of a terror attack or tragic accident, with local police even issuing a statement through Twitter acknowledging that they were looking into these reports. It wasn’t long, however, before it was confirmed that the massive boom was actually created by two Royal Air Force Typhoon fighters going supersonic over the city in order to intercept a wayward, unresponsive aircraft.

While the majority of the fighter jets employed by the militaries of the developed world are capable of achieving supersonic speeds, by and large, they’re barred from doing so over heavily populated areas over concerns about the damage sonic booms can cause to civilian property.

The UK, like the United States, maintains strict control of air traffic over densely populated areas, and in particular, over the nation’s capital. According to statements made by a spokesperson for the Royal Air Force, the fighters were scrambled after an aircraft in London airspace lost communications.

“Typhoon aircraft from RAF Coningsby were scrambled this morning, as part of the UK’s Quick Reaction Alert (QRA) procedures, after an aircraft lost communications in UK airspace,” the spokeswoman said. “The aircraft was intercepted and its communications were subsequently re-established.”

So what really causes a sonic boom? As an aircraft travels below the speed of sound (around 717 miles per hour) the air in front of the aircraft is compressed. When the fighter breaks through the “sound barrier,” it releases that built up pressure in a boom that resembles a thunder clap. Essentially, the boom is the wake of the plane’s sound waves as it exceeds the speed of sound.

Although the FAA has strict rules about where aircraft are permitted to fly at supersonic speeds, it’s not at all uncommon for military aircraft to “go supersonic” in order to quickly intercept potential threats. In 2017, for instance, U.S. Air Force F-15s were authorized to go supersonic in order to intercept a civilian aircraft flying near President Trump’s Mar-a-lago resort after it lost communications with the ground.

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